The present invention relates to the improved structure of a printer head for a dot printer, in particular, relates to the structure of a serial printer which can operate with improved high speed in a high temperature condition.
FIG. 1 shows the principle of the dot matrix printing in a serial printer. A printer head 100 has seven needles for mosaic printing, and travels along a printing line in the direction of the arrow A. During the travelling, needles are selectively drived to hit a paper through an ink ribbon and a desired pattern "A", "B", "C" or "D" is printed. The selection of needles is controlled by the content of an integrated circuit (IC) memory. When the size of a character to be printed is 2.67 mm.times.2.05 mm, 7.times.5 number of dots are enough for printing a recognizable character.
One of the prior needle dot heads for a dot printing process is shown in the U.S. Pat. No. 3,896,918, in which an electro magnetic drive for the operation of printing needles of a mosaic printing head includes a pivotally mounted armature for each needle which are arranged along a circular arc. The construction includes a common yoke for all of the electro magnets which comprise of two concentric cups or walls forming a single unit with cylindrical cores arranged at equal intervals along a circular arc parallel to the genatrix of the cup and located between the individual yoke cups. However, said prior printing head has the disadvantages that the power consumption for driving the needles is large, the size of the apparatus is large, and the operational speed of the printer is rather slow. Those disadvantages come mainly from the fact that a needle is driven by an electromagnet, and all the printing power for striking a piece of paper by a needle is given by said electromagnet.
Another print head for a serial dot matrix printer is shown in U.S. Pat. No. 4,225,250, in which a needle is biased to the first position by a permanent magnet, and balanced at that first position with the force of a spring. When an electromagnet is energized, the flux of the permanent magnet is cancelled, and the needle is moved to the second position by the force of the spring. In this prior art, the printing power for striking a paper by a needle is produced by a spring, not by an electromagnet. Therefore, this printer can be small in size, lower in power consumption, and operate with a relatively high printing speed. However, this printer head has the disadvantage that the printing speed is still not quick enough.
Further, a prior printer head has another disadvantage that printing speed and printing quality in a high temperature condition are not sufficient enough. Those disadvantages come from the decrease of the magnetic flux of a permanent magnet in a high temperature condition. FIG. 2 shows the strength of the magnetic flux of a typical ferrite permanent magnet, in which the vertical axis shows the strength of the flux (energy product) and the horizontal axis shows the temperature. As apparent from FIG. 2, the magnetic flux of a ferrite permanent magnet is considerably reduced in a high temperature condition, and then, the force for attracting a plate spring and/or an armature becomes insufficient in a high temperature condition. The lack of force of a permanent magnet of a printer head provides the undesirable decrease of the printing quality and/or the decrease of the printing speed.
The particular material, like Alnico material, which provides excellent characteristics even in a high temperature condition, is available on the market. However, that material is expensive when compared with ferrite material, and the use of that material would increase the price of a printer head itself.
Other prior printer heads are shown in U.S. Pat. No. 3,659,238 and No. 4,044,668. However, the characteristics of those printer heads are still not sufficient for operation enough in a high temperature condition.
Since the continuous operation of a printer head rises the temperature of the printer head itself, the stable operation at the high temperature is material for a printer head.